He heard the familiar whine of a Metro train approaching the platform, and Tom Baker decided to run for it. The next train was scheduled to arrive at Takoma Station in two minutes, another in six minutes and yet another in 10. But it was the first Monday of summer, and Baker had left work early with a weightlifting routine to complete and an overgrown garden to tend. A doctor at Walter Reed with an emergency pager affixed to his waist, Baker had learned to schedule and protect every minute of his free time. This was his train.

Baker, 47, bounded up the escalator, two steps at once, until he reached the empty platform. The train idled on his left, its doors still open. The operator, a 42-year-old named Jeanice McMillan, stuck her head out the window and watched Baker run toward her. She was an hour into her final shift of the week, seven loops on the Red Line away from going home to a son who just had returned from college. But she smiled and held open the door, and that was how the final passenger made it onto Car 1079, the first car of Metro Train 112, headed south toward downtown Washington.

An automated voice greeted Baker as he slid into a seat directly behind the operator.

"Doors closing."

Train 112: a nondescript Metro train, six cars in all. Car 1079: at least 16 people scattered across 68 seats, lost in their own worlds late on a Monday afternoon. Baker stood up again. If he walked to the rear of the car, he would be closer to his exit at Fort Totten. He would shave nine seconds off his commute home. That seemed important.

Baker tossed his blue backpack over his shoulder and walked the full 75 feet to the back of the car, passing all the other passengers on his way. There was a dentist reading a book about golf; a college student closing his eyes after the fourth day of an internship; a young architect fiddling with his cellphone; a 17-year-old checking her makeup in a small mirror before applying extra lip gloss.

Near the front of the train, a 23-year-old named LaVonda King was on her daily trip to pick up two young sons from day care. She had just finished a cellphone conversation with her mother, who suggested that King print advertising fliers for her new hair salon. A good idea, King agreed. She already had the keys to the shop and a name she had daydreamed about since high school: "LaVonda's House of Beauty."

In the far rear of the car, Dave Bottoms listened to an iPod. A chaplain who had just finished his first day on the pastoral staff at Walter Reed Army Medical Center, Bottoms, 39, felt scattered from the stress of a new job. Wasn't today his dog's seventh birthday? Did his new BlackBerry work? Were there any leftovers in the fridge for a quick dinner? Bottoms reached into his backpack and grabbed a photocopy of a homily by St. Irenaeus. Maybe, Bottoms thought, a little reading would quiet his mind.

Baker stopped walking when he reached the chaplain and stood near him, leaning against a wall by the rear exit of the first car. Baker had moved from Texas to Washington four years ago, bought a downtown condo and sold his car. So liberating. He loved the predictability of Metro. It was 4:57 p.m., and Train 112 lurched into motion, with Car 1079 at the lead. Baker grabbed a pole to steady himself and turned to face the door he planned to use to exit the train. He would make it to the gym by 5:45, probably home by 7:30. A good night ahead. Three minutes to Fort Totten.

* * *

As Baker looked out the window, Bottoms felt the train roll into motion but never bothered to look up from his reading. Ride the Metro long enough and it becomes like sleepwalking: Trees on the right, a blur of graffiti on the left, a subtle bump-bump-bump of the car so predictable that it somehow becomes relaxing. The operator came over the speaker system and said something about a delay, but his iPod muffled her announcement. The train slowed and accelerated, stopped and started, and all the while he kept reading.